Myosin has been isolated from the hearts of human adults, infants, and fetuses of 14-18 weeks gestation. Fetuses and infants express a myosin isoenzyme not seen in adults. We are studying the significance of this isoenzyme by comparing the enzymatic properties of fetal and adult human cardiac myosin, subfragment 1, and heavy meromyosin. We are also studying the role of myosin light chain phosphorylation in the fetal heart.